The National Radio Systems Committee has released a study on the effects of reducing the bandwidth of AM transmissions from the present standard of 10 kHz. The study used representative receivers to present transmissions of speech, sports, music and commercials with bandwidths of 5 kHz, 7 kHz and 10 kHz. The study assumes that both the desired and undesired stations operate using the same standard.
The results showed that mutually reducing transmission bandwidth down to 5 kHz is a big plus for speech. There was either a benefit, or no change from the 10 kHz results when reducing the transmitted bandwidth to 7 kHz irrespective of format or first adjacent channel interference level.
The study was done with a selection of receivers, and used blind A-B testing to determine listener preference. This methodology is superior when the sources are a selection of several transmission bandwidths.
This study assumed symmetry: both the desired signal and the undesired signal were limited to the same bandwidth. This implies a standard to be adopted by all stations within the AM band, which may or may not be a factor for stations that receive interference from stations in other countries.
Since stations broadcasting speech receive a big plus, there will probably be pressure by the “talkers” to adopt the narrower 5 kHz limitation. Â This is also the bandwidth limitation required for medium wave HD radio’s analog signal.
The study did not include the effect of HD radio interference to first adjacent channel stations, and whether an analog station has any benefit from wider transmission bandwidth when an HD signal is the interferor. In the end, HD radio’s medium wave “Digital Only” signal will be an excellent neighbor to other “digital only” and Analog stations, but I expect few stations will be moving to this mode in the next year or so.